Definition
A turbine engine design in which the power turbine that drives the propeller or output shaft is not mechanically connected to the gas-generator turbine. The gas-generator section (compressor and its driving turbine) produces a high-energy gas flow, and that gas flow then spins a separate downstream turbine — the power turbine — which is linked only aerodynamically, not by a shaft, to the rest of the engine.
Plain English
A turboprop or turboshaft engine where the part that makes the hot gas and the part that drives the propeller are on two separate shafts. The hot gas from the first section blows through the second turbine to spin it, but the two are not bolted together.
Context Anchor
Seen in turbine airplane and helicopter systems when describing how engine power is transferred to a propeller, rotor, or gearbox.
Derivation
‘Free’ here means mechanically independent — the power turbine is free to spin at its own speed, not locked to the gas generator. ‘Power turbine’ names its job: extracting the energy from the gas stream and turning it into shaft power for the propeller.
Why Pilots Care
The independent turbine allows smoother power delivery and better propeller control across varying flight conditions without forcing the gas generator to match rotor speed.
Analogy
Think of a stream of air turning a small windmill. The windmill is driven by the moving air, but it is not connected by a shaft to the fan that made the air move.
Intuition Check
Do not read free as meaning loose, uncontrolled, or without limits. Here, free means mechanically separate: the power turbine is driven by gas flow, not by a direct shaft connection to the engine core.
Example Sentence 1
The PT6 is a free power turbine engine, so the propeller can stay almost still at idle while the gas generator continues to run.
Example Sentence 2
During the power check the student confirmed the free power turbine engine was producing rated torque at the selected propeller RPM.